Artificial zinc fluoride solid electrolyte interlayer enabled commercial-level aqueous zn metal batteries

ABSTRACT

A novel conversion-type rechargeable zinc (Zn) battery is constructed. A zinc fluoride (Zn/ZnF 2 ) interface provides an artificial solid electrolyte interlayer (SEI) at the Zn/electrolyte interface that allows plating of Zn dendrite free and protects the Zn metal anode from direct exposure to bulk aqueous electrolyte. This provides the advantage that the interfacial Zn 2+  ion transport is regulated, which controls nucleation distribution and the growth pattern of Zn deposition on the surface of the anode, and also improve inherent H 2 O/O 2  resistant properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional U.S. Application No.63/130,193, filed in the United States Patent and Trademark Office onDec. 23, 2020 entitled, “An Artificial Zinc Fluoride Solid ElectrolyteInterlayer Enabled Commercial-Level Aqueous Zn Metal Batteries”, whichis incorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the field of zinc (Zn) batteries. Inparticular, the present invention relates to a zinc fluoride (ZnF₂)solid electrolyte interlayer (SEI) with high Zn²⁺ transfer number (0.65)to isolate Zn metal from liquid electrolyte

BACKGROUND

Zinc (Zn) batteries are known to deteriorate over time due to formationof dendrites and parasitic hydrogen evolution. The causes of dendriteformation include preferential site deposition on the zinc anodesurface.

Generally, zinc is deposited from the electrolyte onto the battery anodeduring charge cycle. However, if a particular spot on the surface of thezinc anode is deposited with more zinc, that particular spot easilyencourages further deposition to be made over it. Eventually, continualdeposits build up into finger-like zinc structures that extend towardsthe cathode, known as dendrites. Dendrites can short the battery if thedendrites grow toward and reach the cathode.

Up to now, strategies for supressing dendrite-free zinc depositionand/or parasitic hydrogen evolution is mainly limited to electrolyteoptimization, artificial SEI or the fabrication of three-dimensional(3D) current collectors. Although, to some extent, these strategies cansupress dendrites in lab-scale batteries, these strategies are still farfrom being effective in industrial-scale, commercial batteries, and areunable to mitigate unsatisfactory battery performance. Furthermore,these methods are costly.

Therefore, it is desirable to propose a suitable cathode and/or organicelectrolyte that could provide stable, high performing zinc ionbatteries that is less susceptible to dendrite-related performancedeterioration.

STATEMENT OF INVENTION

In a first aspect, the invention proposes a battery comprising: a Znelectrode coated with a layer of ZnF₂ by vapour deposition; and anaqueous electrolyte.

Optionally, the battery further comprises an α-MnO₂ electrode.Preferably, the aqueous electrolyte comprises ZnSO₄.

Optionally, the battery further comprises: a further Zn electrode coatedwith a layer of ZnF₂ by vapour deposition; one Zn electrode being theanode; the further Zn electrode being the cathode; wherein the aqueouselectrolyte is κ m KOH+0.2 m Zn(AC)₂

In a second aspect, the invention proposes a method of making zinccoated with zinc fluoride, suitable for use as an electrode in a aqueouszinc battery, comprising the steps of: providing a piece of zinc;polishing a surface of the piece of zinc; placing the piece of zinctogether with NH₄F powder in a sealed enclosure comprising a vacuumatmosphere; and heating the sealed 180° C. for 12 h.

Optionally, the piece of zinc is a 0.2 mm thick Zn disc with a diameterof 14 mm. Preferably, one side of the Zn disc is carefully polished with1000 mesh sandpaper to remove the passivation layer.

In a third aspect, the invention proposes a Zn@ZnF₂//Zn@ZnF₂ batterycomprising: (a) Two Zn@ZnF₂ electrodes; and (b) 2 M ZnSO₄ aqueouselectrolyte.

Preferably, the cells are cycled at 1, 2 and 5 mA cm⁻² with 1 mAh cm⁻²of the Zn reversibly cycled. The Zn@ZnF₂//Zn@ZnF₂ cells show superiorperformance with a cycle life of >2500 h, where 2500 mAh cm⁻² cumulativecapacity is cycled.

The Zn@ZnF₂//Zn@ZnF₂ battery of claim 9, wherein even at high rate of 10mA cm⁻² with 10 mAh cm⁻² of the Zn reversibly cycled, Zn@ZnF₂//Zn@ZnF₂cell can be operated steadily for ˜590 h without apparent irreversiblevoltage observed.

In yet a further aspect, the invention proposes a Zn@ZnF₂//MnO₂ batterycomprising: (a) A Zn@ZnF₂ electrode; (b) 2 M ZnSO₄+0.2 M MnSO₄ aqueouselectrolyte; and (c) A MnO₂ electrode.

Advantageously, the Zn@ZnF₂//MnO₂ battery (˜3.2 mAh cm⁻²) is capable ofcycling stability over 1000 cycles with 93.63% capacity retained at˜100% coulombic efficiency.

Furthermore, a 850 mAh can operate over 160 cycles (800 h cycling) with93.17% initial capacity retained.

Accordingly, the invention provides the possibility of mitigating theproblem of unstable Zn anode in an aqueous electrolyte, by proposing useof the ZnF₂ as ion conductor.

BRIEF DESCRIPTION OF DRAWINGS

It will be convenient to further describe the present invention withrespect to the accompanying drawings that illustrate possiblearrangements of the invention, in which like integers refer to likeparts. Other embodiments of the invention are possible, and consequentlythe particularity of the accompanying drawings is not to be understoodas superseding the generality of the preceding description of theinvention.

FIG. 1 illustrates schematically a prior art of the invention;

FIGS. 2a-2c illustrate schematically the mechanism in the a prior art ofFIG. 1;

FIG. 3 illustrates schematically an embodiment of the invention;

FIGS. 4a-4d illustrate schematically the mechanism in the embodiment ofFIG. 3;

FIGS. 5a-5d illustrate the production steps for a part of the embodimentof FIG. 3;

FIG. 6 is a photograph of a prototype made according to an embodiment ofFIG. 3;

FIG. 7 is a schematic illustration of the internal structure of theprototype of FIG. 6;

FIG. 8 illustrates schematically an embodiment alternative to that ofFIG. 3;

FIGS. 9a-9b show scanning electron microscope pictures of experimentalobservations on prototypes of made accordingly to the embodiment of FIG.3;

FIGS. 10a-10b show scanning electron microscope pictures of experimentalobservations on prototypes of made accordingly to the embodiment of FIG.3; and

FIG. 11 illustrates the mechanism in the embodiment of FIG. 3.

DESCRIPTION OF EMBODIMENT(S)

FIG. 1 shows an aqueous zinc (Zn) battery 100 according to the priorart. The cathode 101 material is manganese dioxide (a-MnO₂, a mineralalso known as hollandite). The α-MnO₂ cathode is paired with a Zn anode103.

During the discharge cycle of the battery, the Zn anode 103 is oxidisedand produces Zn ions which migrate into the electrolyte solution, andwhich then move towards the cathode 101. FIG. 1 also shows a load 107through which a current of the electrons yielded by the Zn anode 103passes during the battery discharge cycle.

FIGS. 2a-2c shows in greater detail what happens on the surface of theZn anode 103 of FIG. 1 during the discharge cycle. Specifically, FIG. 2ashows a disc representing the anode 103 before the discharging cycle hascommenced. Before the start of the discharging cycle, the volume of theZn electrode is as prepared and placed into the battery. FIG. 2b showshow, as the discharge continues, the anode 103 is slowly consumed andbecomes thinner, because Zn is lost as ions into the electrolyte. Seethickness marked (a) in FIGS. 2a and 2 b.

The Zn ion battery is a secondary battery and can be repeatedlydischarged and re-charged, over a plurality of cycles. FIG. 2cillustrates how, during the charging cycle, reverse Zn migrationhappens. That is, Zn ions in the electrolyte are attracted to, reducedand deposited on the electron-supplying surface of the Zn electrode 103.However, the deposition of Zn is not uniform across the surface of theelectrode. Some parts of the Zn surface is deposited with more Zn thanothers, and these Zn deposits extends into the electrolyte and easilyattract further deposits to form fingers of Zn called dendrites. In anactual battery, the dendrites can grow and extend past a separatorbetween the anode 103 and cathode 101 to cause an internal short circuitin the battery. At the same time, this causes generation of hydrogenwhich bloats the battery.

FIG. 3 shows an embodiment 300 of the invention, and illustrates theschematic battery of FIG. 1, except that the Zn electrode 103 is nowcoated in zinc fluoride (ZnF₂), and is annotated as Zn@ZnF₂. The cathode301 material is α-MnO₂ The electrolyte is aqueous and supplies Zn ions.

The Zn@ZnF₂ notation means that a layer of ZnF₂ as a solid electrolyteinterlayer 303 (SEI) between the elemental Zn forming the anode 103 andthe aqueous electrolyte.

FIGS. 4a-4d show in greater detail what happens on the surface of the Znanode 103 of FIG. 4a during the discharge cycle. FIG. 4a illustratesoriginal Zn electrode having thickness (a) and overlaid with a layer ofZnF₂ 303 before discharging. FIG. 4b shows how the Zn electrode isconsumed as the Zn is oxidised and becomes thinner on being consumed.The Zn recedes away from the ZnF₂ SEI 303 as more zinc ions, Zn²⁺, isproduced.

FIG. 4c shows how Zn ions migrate across the ZnF₂ SEI 303 uniformlyduring the charge cycle, to be deposited onto the other side of the ZnF₂SEI 303 evenly. This layer of Zn deposit on the ZnF₂ grows and extendstowards to the remaining Zn 103 of the original Zn material, reducingthe gap between the layer of ZnF₂ and the original Zn electrode whichhas a remaining thickness (a). Eventually, the layer of Zn on the ZnF₂grows, having thickness (c), and contacts the remaining, original Znmaterial of thickness (a).

Preferably, the ZnF₂ protective layer is uniform in terms of compositionand thickness, which dominates the cation transport. This provides thepossibility of homogenizing transport of Zn²⁺ cations toward theelectrode surface. In contrast, a lumpy layer of ZnF₂ may possiblyencourage dendrite growth.

Accordingly, the ZnF₂ SEI 303 prevents build-up of localised Zn deposit.

The ZnF₂ layer endows a dense and dendrite-free Zn deposition byregulating Zn²⁺ diffusion, controlling nucleation and prohibiting thepermeation of H₂O and O₂. Therefore, the embodiment of FIG. 3 and FIGS.4a-4d produces much less hydrogen gas than the prior art even after manybattery charge and recharge cycles. The reduced hydrogen evolutionprovides the possibility of successful inhibition of side reactions,which improves the reversibility of electrochemical plating/stripping ofmetallic Zn during the respective discharge/charge cycles.

Physically, the electronically insulated ZnF₂ layer segregates to someextent the Zn metal in the electrode from the bulk of the liquidelectrolyte, which reduces charge transfer from the Zn metal to waterH₂O molecules in the electrolyte.

Thereby, chemical oxidation and electrochemical hydrogen evolution atthe Zn@Zn F₂ electrode is restrained.

In other words, the electronically insulated ZnF₂ layer can segregateactive Zn metal from bulk liquid electrolyte and turn off chargestransfer from Zn metal to H₂O molecules of electrolyte, thus restrainingchemical oxidation and electrochemical hydrogen evolution reaction onZn@ZnF₂ electrode.

FIGS. 5a-5d schematically shows how to fabricate an even layer of ZnF₂coating onto a Zn electrode. FIG. 5a shows a piece of Zn foil 501, whichmay be just about 0.2 mm thick, depending on the overall battery design.The surfaces of the Zn foil have been polished with 1000 mesh sandpaperto remove any passivation layer. The polished Zn foil is placed into anenclosure 503, such as a glass tube, with ammonium fluoride (NH₄F)powder 505 placed below the Zn foil 501. Subsequently, the air in thetube is evacuated, at 507, and the tube is sealed using an oxy-acetylenekit. After that, as shown in FIG. 5b , the enclosure is heated to 180°C. for 12 h in a muffle furnace. Upon cooling, as shown in FIG. 5c , theoriginal Zn foil 501 is coated with ZnF₂. The ZnF₂ coated Zn foil 509 isthen removed from the enclosure, as shown in FIG. 5d , ready to be usedin a Zn battery.

The deposited coating is not made up of loose ZnF₂ particles but of auniform coat of deposited ZnF₂. Use of vapour deposition improves theextent of evenness of the ZnF₂ surface. In contrast, if the Zn metalwere coated with ZnF₂ particles, the side-reaction suppression effectwill not be as good.

For completeness, it is mentioned that the cathode 101 is prepared asfollows (not illustrated). MnO₂ is synthesized by a modifiedco-precipitation and hydrothermal method. In a typical procedure, 11mmol Mn(CH3COO)2.4H₂O is dissolved into deionised (DI) water undercontinuous stirring for 0.5 h. Subsequently, the above solution is addeddropwise into an aqueous solution prepared by dissolving 8 mmol KMnO₄into 80 mL DI water and stirring for 0.5 h. The mixed solution is thentransferred to a Teflon-lined autoclave and heated at 120° C. for 12 h.After cooling, the obtained dark brown precipitate is washed severaltimes by DI water and dried at room temperature in a vacuum oven for 8 hto finally obtain the MnO₂ electrode materials. The MnO₂ electrodematerials can be pressed into an electrode layer.

FIG. 6 shows the exterior of a pouch-type battery, which is a prototypeof the embodiment of FIG. 3 and FIGS. 4a-4d and is a high-energymulti-layer pouch-type battery. FIG. 7 illustrates a part of the contentinside the pouch of FIG. 6, which comprises ten whole sets of anode103-separator-cathode 101 stacks with a size of 4*6 cm². Only two suchsets are shown in FIG. 7 for the sake of clarity. The separator ispreferably glass fibre. The battery can be assembled in ambient airenvironment without risk of any complicated procedures and withoutrequirement of special protection, unlike assembling lithium batteries.The pouch is filled with an aqueous electrolyte, illustrated as adroplet, which comprises Zn ions supplying salt, such as zinc sulphateZnSO₄.

Accordingly, the described embodiments provide the possibility of anaqueous Zn battery that is capable of reduced dendrite formation andreduced hydrogen evolution because of the SEI of ZnF₂, compared to Znbatteries of the prior art.

FIG. 8 shows yet another embodiment, wherein the both the anode 103 andthe cathode 801 are Zn electrodes coated with ZnF² 303, 803, i.e. aZn@ZnF₂//Zn@ZnF₂ battery. The electrolyte is an alkaline electrolyte 6 MKOH+0.2 m Zn(AC)₂. In this embodiment the electrons simply flow from oneelectrode to the other through every charge and discharge cycle. Thelayer of ZnF₂ provides the possibility that both Zn electrodes remainthe same size without shape distortion, and without dendrite formationfor a longer period of time compared to what is possible with the priorart electrode.

Experimental Observations

The morphologies of Zn@ZnF₂ and bare Zn electrodes have been analysedafter they were subjected to 50 cycles of stripping/plating(discharge/charge) in symmetric cells at 2 mA cm⁻² with 1 mAh cm⁻² ofthe Zn reversibly cycled.

The scanning electron microscopy (SEM) image in FIG. 9a shows a smoothand dense surface of deposited Zn, while the ZnF₂ layer remainingessentially flat even after many cycles, with no obvious dendrites. Incontrast, FIG. 9b shows dendrite deposits on a bare Zn electrode merelyafter a few charge cycles. The dendrites formed consist of thinplatelets and are highly porous.

FIGS. 10a-10b shows a cross-sectional SEM image. FIG. 10a shows thestripped state (discharged state) of the Zn@ZnF₂ electrode, in which aclear gap is visible beneath the ZnF₂ layer FIG. 10b shows the same theelectrode in the plated state after a charging cycle, in which no gap isvisible. FIG. 10b also shows that Zn deposits, which have grown on theside of the ZnF₂ layer facing the original zinc metal, have come intocontact with the original zinc metal.

FIG. 11 shows the initial state (IS), transition state (TS), and finalstate (FS) structures of Zn2⁺ diffusion through the ZnF₂ (002) surface.The Zn and F atoms are shown as the criss-cross structure, and theintercalated Zn atom is shown diffusing inside the criss-cross structurefrom the IS, to the TS to the FS state.

The ZnF₂ comprises geometrically optimized F element which candistinctly induce charge transfer and redistribution at the Zn/ZnF₂interface. Positive charge accumulates around Zn layer and negativecharge distributes around Zn—F bonds. This can promote the formation ofsubstantial Zn@ZnF₂ interface and fast Zn²⁺ diffusion on the surface.

The energy barrier for Zn²⁺ diffusion on ZnF₂ (002) surface iscalculated to be 0.27 eV, while Zn²⁺ hopping in bulk ZnF₂ represent abarrier as high as 0.76 eV determined indicating that exposed (002)surface can greatly enhance the Zn²⁺ diffusion.

The lower energy barrier of Zn²⁺ inserted into Zn@ZnF₂ can be validatedby the decrease in Gibbs free energy (ΔG), where the ΔG obtained forZn@ZnF₂ is 0.52 eV, lower than that of Zn metal (0.67 eV). Accordingly,the Zn@ZnF₂ preferentially provides electrostatic attraction towardZn²⁺, accelerating the kinetics through reducing the deposition barrier.

During electrochemical process, the Zn deposition only comes up at aplace, where the Zn²⁺ forgather charges. Therefore, the ZnF₂ play a roleof regulating the transport of Zn²⁺ cations towards the electrodesurface, without any Zn deposition on ZnF₂ surface.

The Zn²⁺ transport flux could be dominated by ZnF₂ SEI during theelectrodeposition. Moreover, the Zn@ZnF₂ electrode shows very low chargetransfer resistance of 39.99 ohm, a tenth lower than that of the bare Znelectrode (412.62 ohm).

The Zn²⁺ transfer number of the ZnF₂ reaches to 0.65, which is muchlarger than that of GF separator coupled with 2 M ZnSO₄ aqueouselectrolyte (0.28). The above results suggest outstanding Zn²⁺transportation capability of the ZnF₂ layer.

Accordingly, he ZnF₂ layer does not promote isolated Zn nucleation, andthe Zn deposit tends to grow on the Zn@ZnF₂ interface with lowerinserting energy. Consequently, Zn²⁺ ions is deposited into the gapbetween the ZnF₂ layer and the original Zn foil, and Zn deposition canbe confined to the underside of the ZnF₂ layer, and thus dendrite growthis suppressed.

The ZnF₂ is highly permeable to Zn²⁺ ions, as Zn²⁺ ions are not trappedand retained in the ZnF₂ lattice. The energy barrier of inserting Zn²⁺into the zinc electrode overlaid with ZnF₂, i.e. the Zn@ZnF₂ lattice,has been observed to be relatively low. That is, the insertion barrierenergy into Zn@ZnF₂ is just 0.52 eV, lower than the insertion barrierenergy into ZnF₂ (4.08 eV) and the insertion barrier into metallic Znsurface (0.67 eV). Accordingly, the Zn@ZnF₂ provides electrostaticattraction to Zn²⁺, which can accelerate the kinetics of passage,reducing the deposition barrier.

During the charging cycle, ZnF₂ is capable of directing the flux of Znions that flows towards the Zn surface in a laminar manner, generallywell spread across the ZnF₂. In the prior art, Zn ions are moreattracted towards the parts of the surface of the Zn electrode that hasmore charge which creates preferential deposits on parts of the Znsurface, causing dendrite. In the present embodiment, as ZnF₂ is in theway of the ion flow, the ZnF₂ acts as a screen that preventsaccumulation of Zn²⁺ cations to towards the electrode surface. Thisreduces localised electrodeposits and dendrites.

The Zn²⁺ transfer number of ZnF₂ can be as high as 0.65, which is muchlarger than that of the glass fibre separator, in embodiments that havea 2 m ZnSO₄ aqueous electrolyte (0.28). Therefore, the ZnF₂ layerprovides the possibility of relatively good Zn²⁺ transportation.

Also, it has been found that Zn@ZnF₂ electrode has a low charge-transferresistance of 39.99Ω, a tenth lower than that of the bare Zn electrodeat 412.62Ω.

Over many cycles of discharging and charging, the plating/strippingover-potential for Zn@ZnF₂ is only 56.4 mV, which is significantly lowerthan that of bare Zn (238.9 mV). This provides excellent reversibilityand cyclic lifespan that with good suppression of dendrite and hydrogenevolution. The outstanding reversibility of Zn deposition/dissolutionwith low overpotential originates from high ionic conductivity and fastion diffusion of ZnF₂ solid in Zn²⁺-ion conductor as well as goodstability of ZnF₂ layer in aqueous environment.

It has been found that a battery of this embodiment can be cycled over400 h at 2 mA cm⁻² and 140 h at 5 mA cm⁻² with 1 mAh cm⁻² of Znreversibly cycled in an alkaline electrolyte of (6 m KOH+0.2 m Zn(AC)2).In sharp contrast, after only <10 h plating/stripping cycles with Zn//Zncell in the same condition, a sudden polarization occurs due tointensified Zn-dendrite formation. Accordingly, the describedembodiments provide the possibility of superior reversible cyclingperformance using either mild or alkaline electrolytes.

In contrast, Zn@ZnF₂ electrode coated with commercial ZnF₂ powders,instead of being coated a flat layer of ZnF₂ deposit, induces fastdendrite formation and growth, leading to only ≈290 h cycling stabilityof symmetric cells in mild electrolyte, much lower than bare Znelectrode-based cells (600 h).

Accordingly, the embodiments described provide the possibility ofproducing aqueous secondary zinc batteries capable of being dischargedand recharged.

Also, the embodiments include a compact and homogeneous zinc fluoride(ZnF₂) SEI layer utilizing an in-situ ion metathesis method. At theZn/ZnF₂ interface, the F atoms of ZnF₂ layer tightly bond with Zn atomsof Zn metal, giving the credit to the charge migration between Zn and Fatoms and charge redistribution. Meanwhile, the Zn@ZnF₂ interfaceexhibits a much higher ΔGH value for hydrogen evolution side reaction.

The Zn/ZnF₂ interface provides an artificial SEI at the Zn/electrolyteinterface that allows plating of Zn dendrite free and protects the Znmetal anode from direct exposure to bulk aqueous electrolyte. Thisprovides the advantage that the interfacial Zn²⁺ ion transport isregulated, which controls nucleation distribution and the growth patternof Zn deposition on the surface of the anode, and also improve inherentH₂O/O₂ resistant properties.

As a result, the obtained composite (denoted as Zn@ZnF₂) enableslong-term Zn dendrite-free plating/stripping and prevent the fresh Znmetal from contacting with H₂O molecular in the bulk electrolyte,avoiding side reactions of hydrogen evolution and Zn corrosion.Consequently, the dendrite-free and side reaction-free Zn anode enableZn@ZnF₂//Zn@ZnF₂ symmetric cell cycling over 2500 h even at high currentdensity of 5 mA cm⁻² with a areal capacity of 1 mAh cm⁻² andcommercial-level Zn@ZnF₂//MnO₂ full cell operates 1000 cycles with93.63% capacity retained even at high areal capacity of 3.2 mAh cm⁻².Finally, an 850 mAh Zn@ZnF₂//MnO₂ large-capacity battery exhibitsexcellent cycling stability over 160 cycles with 93.17% capacityretention, far outperforming Zn//MnO₂ (failed after only 15 cycles) insame condition.

1. A battery comprising: a Zn electrode coated with a layer of ZnF₂ byvapour deposition; and an aqueous electrolyte.
 2. An battery as claimedin claim 1, further comprising: an α-MnO₂ electrode.
 3. An battery asclaimed in claim 2, wherein the aqueous electrolyte comprises ZnSO₄. 4.An battery as claimed in claim 1, further comprising: a further Znelectrode coated with a layer of ZnF₂ by vapour deposition; one Znelectrode being the anode; the further Zn electrode being the cathode;wherein the aqueous electrolyte is 6 m KOH+0.2 m Zn(AC)2.
 5. A method ofmaking zinc coated with zinc fluoride, suitable for use as an electrodein a aqueous zinc battery, comprising the steps of: providing a piece ofzinc; polishing a surface of the piece of zinc; placing the piece ofzinc together with NH₄F powder in a sealed enclosure comprising a vacuumatmosphere; and heating the sealed 180° C. for 12 h.
 6. A method asclaimed in claim 5, wherein the piece of zinc is a 0.2 mm thick Zn discwith a diameter of 14 mm.
 7. A method as claimed in claim 6, wherein oneside of the Zn disc is carefully polished with 1000 mesh sandpaper toremove the passivation layer.